Top-emitting white organic light-emitting diodes having improved efficiency and stability

ABSTRACT

The present disclosure relates to an emissive construct, which can be used in various OLED applications, for example, top-emission white organic light-emitting diodes. The emissive construct includes a fluorescent emissive layer, a partial hole-blocking layer, and a phosphorescent emissive. A recombination zone is shared between the fluorescent emissive layer and the phosphorescent emissive layer, such that the thickness of the partial hole-blocking layer is less than about one-third of the thickness of the recombination zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/570,667, filed Dec. 14, 2011, which is incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to top-emission white-color organiclight-emitting diode (OLED) devices for lighting applications.

2. Description of the Related Art

Organic light-emitting materials offer a very promising field of studyfor energy efficient lighting applications. Many methods have beenproposed to increase the OLED device power efficiency, includingmodifying materials, device structure, device fabrication techniques,and light outcoupling techniques. A traditional OLED comprises a bottomemission type OLED (BE-OLED), wherein the bottom electrode is atransparent conducting metal oxide, such as Indium-Tin-Oxide (ITO)deposited on top of a transparent substrate, such as glass. Generally,without light outcoupling involved, most of the emitted light in aBE-OLED is trapped inside the device in the form of an organic mode,substrate mode, or plasma mode. Only about 10-30% of the light escapesfrom the device and contributes to the lighting. Thus, the light trappedin the glass substrate may account for 20% of the total emissive light.This generally requires light extraction in BE-OLEDs to be practicallynecessary.

Recently, top-emission OLED (TE-OLED) devices, wherein the top electrode(generally, the cathode) is either a semi-transparent metal cathode or atransparent conducting metal oxide like ITO, have been explored. For asemi-transparent top cathode, the microcavity effect may be serious dueto a relatively higher reflectance of the metal semi-transparent cathodecompared with a transparent ITO cathode. This can lead to selectivewavelengths passing through the cathode, contributing to the lightoutput and viewing angle dependence of the emission spectrum. While sucha feature may be good for display applications, it can also negativelyaffect general lighting applications because white-color light emissionis desired.

There are many challenging issues in TE-OLED manufacturing, includingmaterials for the bottom reflective anode, the active cells of thelight-emitting layers, and the semi-transparent cathode. Also, tuningthe light enhancement layer and the light scattering layer, all whilefurther enhancing the power efficiency of TE-OLED to meet variouslighting application requirements, invokes large amounts ofconsideration. Compared to BE-OLED, the efficiency needs of TE-OLEDrequire much more attention in order to meet the light applicationrequirement.

SUMMARY OF THE INVENTION

An embodiment provides an emissive construct, which can be used invarious OLED applications, for example, top-emission white organiclight-emitting diodes. In some embodiments, the emissive constructcomprises a fluorescent emissive layer comprising a first host material,a partial hole-blocking layer having a first thickness disposed on thefluorescent emissive layer, and a phosphorescent emissive layer disposedon the partial hole-blocking layer, comprising a second host material.In some embodiments, a recombination zone is shared between thefluorescent emissive layer and the phosphorescent emissive layer. Insome embodiments, the recombination zone has a second thickness, whereinthe first thickness of the partial hole-blocking layer is less thanabout one-third of the second thickness of the recombination zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the layers of a white TE-OLED device.

FIG. 2 shows the electroluminescence (EL) spectrum of an embodiment of awhite TE-OLED device at lower (2000 nit) and higher (10000 nit)brightness levels.

FIG. 3 shows the brightness dependence of current efficiency and powerefficiency of an embodiment of a white TE-OLED device.

FIG. 4 shows the brightness level over the lifetime of an embodiment ofa white TE-OLED device.

FIG. 5 shows the brightness level over the lifetime of an embodiment ofanother white TE-OLED device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The emissive constructs described herein can be used in various devices.In some embodiments, the emissive construct is used in an OLED. In someembodiments, the OLED comprising the emissive construct is selected froma BE-OLED or a TE-OLED. Preferably, the OLED comprising the emissiveconstruct comprises a top-emission white OLED. OLEDs can be constructedof various known layers. In some embodiments, the OLED comprises ananode and a cathode. In some embodiments, the anode comprises areflective anode. In some embodiments, the cathode comprises asemi-transparent or transparent cathode.

The layers that comprise emissive construct may be positioned in thedevice at various locations, though preferred embodiments are furtherdescribed below. Preferably, the emissive construct comprises afluorescent emissive layer, a partial hole-blocking layer adjacent tothe fluorescent emissive layer, and a phosphorescent emissive layeradjacent to the partial hole-blocking layer. Furthermore, additionallayers may also be present. For example, the OLED may comprise asubstrate. In some embodiments, the OLED comprises an insulating layer.In some embodiments, the OLED comprises a hole-injection layer. In someembodiments, the OLED comprises a hole-transport layer. In someembodiments, the OLED comprises an electron transporting layer. In someembodiments, the OLED comprises an electron injection layer. In someembodiments, the OLE comprises a light emission enhancement layer. Insome embodiments, the OLED comprises a light scattering layer.

Each of the layers in the OLED can be present in any order from bottomto top. Where a first layer is disposed over a second layer, the firstand second layers can be, but need not be adjacent to one another. Wherea first layer is disposed on a second layer, then the first layer isadjacent to the second layer. In some embodiments, the insulating layeris disposed over the substrate. In some embodiments, the reflectiveanode is disposed over the insulating layer. In some embodiments, thehole-injection layer is disposed over the reflective anode. In someembodiments, the hole-transport layer is disposed over thehole-injection layer. In some embodiments, the emissive construct isdisposed over the hole-transport layer. In some embodiments, theelectron transporting layer is disposed over the emissive construct. Insome embodiments, the electron injection layer is disposed over theelectron transporting layer. In some embodiments, the semi-transparentor transparent cathode is disposed over the electron transport layer. Insome embodiments, the light emission enhancement layer is disposed overthe semi-transparent or transparent cathode. In some embodiments, thelight scattering layer is disposed over the light emission enhancementlayer. Any layer that is disposed over another layer may or may not beadjacent to that other layer.

FIG. 1 depicts an example of a light-emitting device 100. In thisembodiment, the light emitting device is a top-emitting white OLED,which emits light from the cathode side. The device comprises atransparent or opaque substrate 101, an insulating layer 102 disposedover the transparent or opaque substrate 101, a reflective or opaqueanode 103 disposed over the insulating layer 102, a hole-injection layer(HIL) 104 disposed over the reflective or opaque anode 103, ahole-transport layer (HTL) 105 disposed over the HIL 104, and anemissive construct 110 disposed over the HTL.

In FIG. 1, the emissive construct 120 comprises three layers. First, afluorescent emissive layer 121 comprising a first host material isdisposed over the HTL 105. A partial hole-blocking layer 122 having isdisposed over the fluorescent emissive layer 121 and a phosphorescentemissive layer 123 is disposed over the partial hole-blocking layer 122.A recombination zone includes the fluorescent emissive layer 121 and thephosphorescent emissive layer 123. The recombination zone is the areashared among the complimentary emissive layers where positive andnegative charges are combined. In some embodiments, the fluorescentemissive layer 121 and the phosphorescent emissive layer 123 define therecombination zone. In some embodiments, the partial hole-blocking layer122 has a thickness that is less than about one-third of the thicknessof the recombination zone. The thickness of the recombination zone isdefined by the combined thickness of all the emissive layers (e.g.,fluorescent emissive layer 121 and the phosphorescent emissive layer123).

As shown in FIG. 1, an electron-transport layer (ETL) 106 is disposedover the emissive construct 120, an electron injection layer (EIL) 107is disposed over the ETL 106, a semi-transparent or transparent cathode108 is disposed over the ETL 106, a light emission enhancement layer 109is disposed over the semi-transparent or transparent cathode 108, and alight scattering layer 110 is disposed over the light emissionenhancement layer 109.

In some embodiments, the fluorescent emissive layer 121 comprises afirst host material. Various host materials can be utilized. Forexample, the first host material may be a fluorescent material, such asa blue light-emitting fluorescent material, that is capable offluorescence without any fluorescent dopant. The fluorescent materialmay also be any material that is suitable as a host for a fluorescentdopant material. In some embodiments, the first host emits blue light.In some embodiments, the fluorescent emissive layer is undoped. Forexample, the first host material may include, but is not limited to, oneor more of the following compounds:

In some embodiments, the host comprises a non-polymeric compound. Insome embodiments, the host consists essentially of a non-polymericcompound. Compounds described in US 20110062386 and U.S. ProvisionalPatent Application No. 61/426,259, filed Dec. 22, 2010, both of whichare incorporated by reference in their entirety, may also be used asfirst host materials.

In some embodiments, the fluorescent emissive layer comprises a bluelight-emitting fluorescent dopant. In some embodiments, theblue-emitting fluorescent dopant materials have a T1 that is greaterthan about 2.3. For example, the blue light-emitting fluorescent dopantcan be selected from the following compounds. The terms “T1,” or“triplet energy,” have the ordinary meaning understood by a person ofordinary skill in the art, and include the energy of the transition fromlowest energy triplet state of an exciton to the ground state. There aremany methods known in the art that may be used to obtain the tripletenergy, such by obtaining phosphorescence spectrum.

STRUCTURE T₁ Vlaue

2.34

2.35

2.38

2.38

2.36

2.37

In some embodiments, the first host material has a S1energy level thatis higher than a S1energy level of the blue light-emitting fluorescentdopant. As used herein, “S1” refers to the lowest energy excited singletstate of an exciton. As used herein, an “exciton” refers to molecule, anatom, or an associated group of molecules and/or atoms in an excitedelectronic state. A higher energy S1 of the first host material mayallow an exciton of the host material to more readily transfer excitedsinglet energy to a lower S1energy fluorescent dopant, as compared to adopant that has a higher S1energy than the first host material.Transferring excited singlet energy to the dopant provides a dopant inthe S1state, which can then fluoresce.

In some embodiments, the Host S1is greater than the fluorescent blueemitter S1. In some embodiments, the Host T1 is greater than thephosphorescent blue emitter T1. In some embodiments, the Host T1 is lessthan the fluorescent blue emitter T1.In some embodiments, the first hostmaterial has a T1 that is lower than a T1 of the blue light-emittingfluorescent dopant. In some embodiments, the first host material has asinglet energy (S1) that is higher than a singlet energy (S1) of theblue light-emitting fluorescent dopant.

In some embodiments, the phosphorescent emissive layer is anorange-emitting layer. In some embodiments, the phosphorescent emissivelayer comprises a second host material. Various host materials can beutilized in the phosphorescent emissive layer. In some embodiments, thesecond host in the phosphorescent emissive layer is the same as thefirst host in the fluorescent emissive layer. For example, the secondhost may comprise any of the compounds listed as options for the firsthost above. The second host may also be different than the first host.In some embodiments, the T1 of the first host in the fluorescentemissive layer is greater than the T1 of the second host in thephosphorescent emissive layer.

The phosphorescent emissive layer may also include one or morephosphorescent dopants. For example, the phosphorescent emissive layermay comprise a second host and one or more phosphorescent dopants, suchas one or more phosphorescent dopants that are (1) yellow and redemitters, (2) green and red emitters, or (3) a single orange emitter. Insome embodiments, the T1of the phosphorescent second host is greaterthan the T1of the one or more phosphorescent dopants.

In some embodiments, the phosphorescent emissive layer comprises amaterial or materials that emit(s) a complementary color light, suchthat the blue light emitted from the fluorescent blue emitting layercombines in whole or in part with the phosphorescent emission of thephosphorescent emissive layer to provide a perceived white light. Insome embodiments, the second host of the phosphorescent emissive layercan be selected from the following compounds:

In some embodiments, the phosphorescent emissive layer may be awhite-light creating complementary phosphorescent emitter. In someembodiments, the phosphorescent emissive layer can be a yellow emittingcompound and a red emitting compound. In some embodiments, the yellowemitting compound can be YE-01.

In some embodiments, the red emitting compound can be Ir(piq)2acac.

Other appropriate complementary emitters can be selected from thosedescribed in U.S. patent application Ser. No. 13/293,537, filed Nov. 10,2011 and U.S. Provisional Patent Application Nos. 61/449,032, filed Mar.3, 2011, and 61/533,679, filed Sep. 12, 2011, the contents of each ofwhich are incorporated by reference herein in their entirety.

The thicknesses of the fluorescent emissive layer and the phosphorescentemissive layer may vary. In some embodiments, the thickness of thefluorescent emissive layer is in the range of about 5 nm to about 50 nm.In some embodiments, the thickness of the fluorescent emissive layer isin the range of about 10 nm to about 50 nm. In some embodiments, thethickness of the fluorescent emissive layer is in the range of about 10nm to about 40 nm. In some embodiments, the thickness of the fluorescentemissive layer is in the range of about 10 nm to about 30 nm. In someembodiments, the thickness of the phosphorescent emissive layer is inthe range of about 5 nm to about 50 nm. In some embodiments, thethickness of the phosphorescent emissive layer is in the range of about10 nm to about 50 nm. In some embodiments, the thickness of thephosphorescent emissive layer is in the range of about 10 nm to about 40nm. In some embodiments, the thickness of the phosphorescent emissivelayer is in the range of about 10 nm to about 30 nm. In someembodiments, the fluorescent emissive layer has a thickness of about 20nm. In some embodiments, the phosphorescent emissive layer has athickness of about 20 nm.

A partial hole-blocking layer is disposed between the fluorescentemissive layer and the phosphorescent emissive layer, acting asconfinement of electron-hole recombination center, exhibitingcolor-stability with respect of applied voltage. Additionally, arecombination zone is shared between the fluorescent emissive layer andthe phosphorescent emissive layer. Various materials can be used in thepartial hole-blocking layer. Preferably, the T1of the partialhole-blocking layer material is greater than both the T1of thefluorescent host material and the T1of the phosphorescent host material.In some embodiments, the HOMO value of the partial hole-blocking layeris more negative than the HOMO value of the fluorescent host material.

In some embodiments, the partial hole blocking layer can include, forexample, materials having an2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD),1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole (OXD-7),1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene (BPY-OXD),2,9-dimethyl-4,7-diphenyl-phenanthroline (bathocuproine or BCP), and1,3,5-tris[2-N-phenylbenzimidazol-z-yl]benzene (TPBI). In someembodiments, the partial hole-blocking layer allows about 50% to about95% of the holes reaching the hole blocking layer to pass from thephosphorescent emissive layer to the fluorescent emissive layer. In someembodiments, the partial hole-blocking layer allows about 50% to about95% of the holes reaching the hole blocking layer to pass from thefluorescent emissive layer to the phosphorescent emissive layer.

Preferably, the partial hole-blocking layer has a thickness that is lessthan about one-third of the thickness of the recombination zone. In someembodiments, the thickness of the partial hole-blocking layer is in therange of about 0.5 nm to about 3 nm. In some embodiments, the thicknessof the recombination zone is in the range of about 2 nm to about 40 nm,about 2 nm to about 30 nm, or about 2 nm to about 20 nm. In someembodiments, the thickness of the recombination zone is in the range ofabout 5 nm to about 15 nm. In some embodiments, the thickness of therecombination zone is in the range of about 8 nm to about 12 nm. In someembodiments, the thickness of the recombination zone is about 10 nm.

Another embodiment provides a method for color tuning a top-emissionwhite organic light-emitting diode. In some embodiments, the methodcomprises inserting a partial hole-blocking layer having a firstthickness, as described herein, between a fluorescent emissive layer anda phosphorescent emissive layer, and adjusting the first thickness totune the color of the top-emission white organic light-emitting diode.The fluorescent emissive layer and the phosphorescent emissive layercombined has a second thickness. In some embodiments, the methodcomprises thickening the hole-blocking layer (increasing the firstthickness) to provide a blue shift. In some embodiments, the methodcomprises thinning the hole-blocking layer (decreasing the firstthickness) to provide a red shift.

The cathode layer can be a semi-transparent metal electrode comprisingmetal alloys (such as Mg:Ag mixture), a bi-layer structure (such asCa/Au), or a transparent electrode (such as ITO, Al:ZnO). The cathodecan also comprise transparent and conducting carbon materials (such asCNT, grapheme). The light enhancement layer can comprise transparentmaterials, which may comprise both organic small molecule materials andinorganic materials including metal oxide, or wide band gapsemiconductor compounds (band gap larger than blue light, wavelengthshorter than 450 nm). The light scattering layer may comprise a thermaldeposited porous nano-structured film.

The hole injection layer can comprise transition metal oxide. In someembodiments, the reflective-opaque anode comprises an Ag and Albi-layer. The hole-transport layer can be partially p-doped and theelectron-transport layer can be partially n-doped. The partially dopedmeans there are still certain thickness of the transport layer undopedclose to the original layer.

In some embodiments, a white light emitting OLED device is providedwhich can include, in sequence from bottom to top, a substrate, aninsulating layer coated on top of the substrate; a reflective and opaqueanode above the insulating layer; a hole injection layer above theanode; a hole transport layer above the hole injection layer; theemissive construct described above; an electron transporting layer abovethe emissive construct; an electron injection layer above the electrontransporting layer; a semitransparent or transparent cathode above theelectron transport layer, a light emission enhancement layer above thecathode; and a light scattering layer disposed above the light emissionenhancement layer. The materials of the substrate, the insulating layer,the reflective and opaque anode, the hole injection layer, the holetransport layer, the electron transporting layer, the electron injectionlayer, the semitransparent or transparent cathode, the light emissionenhancement layer, and the light scattering layer are further describedin U.S. Provisional Patent Application No. 61/533,679, filed Sep. 12,2011, which is further incorporated by reference in its entirety herein,particularly for the discussion of these types of OLED layers.

For lighting application, top-emission white organic light emittingdiode has the issue of lower efficiency and color changing with viewingangles, and complex device structure. This invention, consideringoverall device design and materials selected for each layer of thedevice, solved these issues and achieved: simple device structure, easyprocessing, all the device fabrication done through thermal deposition,new world record in the device power efficiency with white color meetingthe DOE general lighting requirement and insensitive color respect todifferent viewing angles.

In some embodiments, the invention may provide a method for color tuninga white light emitting hybrid OLED device emit a colder (more blue)light which can include inserting the emissive construct described abovebetween an anode and a cathode; and thickening the HBL layer asufficient distance to provide the desired blue shift.

In some other embodiments, the invention may provide a method for colortuning a white light emitting hybrid OLED device to emit a warmer (morered/orange light) light comprising inserting the emissive constructdescribed above between an anode and a cathode; and adjusting the HBLlayer a sufficient distance (thickness) to provide the desired blue/redshift.

Some embodiments provides an emissive construct, which can be used invarious OLED applications, for example, top-emission white organiclight-emitting diodes. In some embodiments, the emissive constructcomprises a fluorescent emissive layer comprising a first host material,a partial hole-blocking layer having a first thickness disposed on thefluorescent emissive layer, and a phosphorescent emissive layer disposedon the partial hole-blocking layer, comprising a second host material.In some embodiments, a recombination zone is shared between thefluorescent emissive layer and the phosphorescent emissive layer, and isdefined as including the fluorescent emissive layer and thephosphorescent emissive layer. The thickness of the recombination zoneis defined as the combined thickness of the fluorescent emissive layerand the phosphorescent emissive layer. In some embodiments, therecombination zone has a second thickness, wherein the first thicknessof the partial hole-blocking layer is less than about one-third of thesecond thickness.

In some embodiments, the phosphorescent emissive layer comprises aphosphorescent dopant, wherein the light emitted by the phosphorescentdopant provides white light when combined with the light emitted by thefluorescent emissive layer. In some embodiments, the phosphorescentemissive layer comprises a yellow-emitting phosphorescent dopant and ared-emitting phosphorescent dopant. In some embodiments, both thefluorescent emissive layer and the phosphorescent emissive layer areabout 20 nm thick. In some embodiments, a highest occupied molecularorbital of the hole-blocking layer has a higher energy than a highestoccupied molecular orbital of the first host material.

In some embodiments, the yellow-emitting phosphorescent dopant and thered-emitting phosphorescent dopant do not need to be included in thesame phosphorescent emissive layer. In those embodiments, thephosphorescent emissive layer of the emissive construct may furthercomprise two sub-layers. The first sub-layer comprises a yellow-emittingphosphorescent dopant and the second sub-layer comprises a red-emittingphosphorescent dopant, or vice versa. In some embodiments, the host ofeach of the sub-layers may be the same. In some embodiments, the twosub-layers may have different hosts.

EXAMPLES

It has been discovered that embodiments of top-emission while OLEDsproduced using the systems and methods disclosed above can achievesimple device structure and easier processing. The OLEDs can bemanufactured using thermal deposition and provide improved device powerefficiency. These benefits are further shown by the following examples,which are intended to be illustrative of the embodiments of thedisclosure, but are not intended to limit the scope or underlyingprinciples in any way.

Example 1 Device Fabrication

Pre-cleaned glass substrates were baked at about 200° C. for about 1hour under ambient environment, then under UV-ozone treatment for about30 minutes. Then, a poly methyl methacrylate (PMMA) layer (about 180 nmthick) was spin-coated on top of the surface of the glass substrates(solution: 2 wt % PMMA in di-chloro benzene [DCB] solvent) at about 6000RPM for about 40 seconds. The substrates were then baked at about 120°C. for about 2 hours. The substrates were loaded into a depositionchamber. A bi-layer reflective bottom anode, (50 nm Al layer and 50 nmAg layer) was deposited sequentially, first Al then Ag, at a rate ofabout 2 Å/s. Molybdenum oxide (MoO₃, about 10 nm) was deposited as ahole-injecting layer at deposition rate of about 1 Å/s. Then a p-dopinglayer (20 nm), MoO₃ was co-deposited with4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (NPB) at 5% in volumeratio at the deposition rate of about 0.051 Å/s and about 1 Å/s for MoO₃and NPB, respectively. A layer of NPB (about 20 nm) was then depositedas a hole-transport layer. A fluorescent blue emissive layer (20 nm) wasthen deposited having a fluorescent blue emitter (BE-1) that wasco-deposited with a host material (Host-1) at 10% in volume with thedeposition rate of about 0.1 Å/s for BE-1 and about 1 Å/s for Host-1.

Then, a partial hole blocking layer of1,3,5-Tris(1-phenyl-1H-benzimidazol-)2-yl)benzene (TPBI) was depositedon top of the fluorescent blue emissive layer at about 0.1 Å/s for athickness of about 2 nm. Then deposition of the phosphorescent emissivelayer (20 nm) of three co-deposition of host (Host-1) with yellowemitter (YE-1) and red emitter (Ir(pq)₂acac) at the deposition rate ofabout 1 Å/s for Host-1, about 0.05 Å/s for YE-1, and about 0.005 Å/s forIr(pq)₂acac.

The doping concentration of the yellow emitter and the red emitter wereabout 5% and about 0.5% by volume, respectively. Next, an electrontransport layer (ETL) of about 30 nm was deposited at the depositionrate of about 1 Å/s. The electron injection layer (EIL) was thendeposited as a thin layer of lithium fluoride (LiF, 1 nm thick,deposition rate 0.1 Å/s) and a thin layer of magnesium (Mg, 1 nm thick)at about 0.1 Å/s. A semi-transparent cathode (about 21 nm) was depositedby co-deposition of magnesium (Mg) and silver (Ag) at a ratio of about1:2 by volume. A light enhancement layer of MoO₃ (70 nm) was depositedon top of the cathode. Finally a light scatter layer(3,5-bis(3-(benzo[d]oxazol-2-yl)phenyl)pyridine) was deposited on top ofthe light enhancement layer at deposition rate of about 2 Å/s for 900nm. All the deposition was done at a base pressure of about 2×10⁻⁷ torr.The device area was approximately 7.7 mm².

The electroluminescence spectrum of Example 1 was measured. FIG. 2 showsan EL spectrum of the TE-WOLED of current invention at lower (2000 nit)and higher brightness (10000 nit) with CIE(0.44, 0.36), CRI(65). Asshown in FIG. 2, the hole-blocking layer effectively confines the chargerecombination center at the interface between the orange and blueemissive layers, giving stable emissive color at higher brightness.

The brightness dependence of the current efficiency and power efficiencyof Example 1 was also measured. FIG. 3 shows the brightness dependenceof current efficiency and power efficiency of an embodiment of a whiteTE-OLED device.

FIG. 4 shows the brightness level over the lifetime of a device inaccordance with Example 1, except the substrate was PEDOT coated withITO/Glass. FIG. 5 shows the brightness level over the lifetime ofExample 1. As shown in FIGS. 4 and 5, the device lifetime and stabilityis improved using the more simplified substrate of PMMA coated withglass.

Although the subject matter of the claims have been disclosed in thecontext of certain embodiments and examples, it will be understood bythose skilled in the art that the scope of the claims extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. Thus, it is intended that the scope of the present claimsshould not be limited by the particular disclosed embodiments describedabove.

What is claimed is:
 1. A top-emission white organic light-emitting diode(OLED) comprising an emissive construct, wherein the emissive constructcomprises: a fluorescent emissive layer comprising a first host materialthat emits blue light, wherein the fluorescent emissive layer isundoped; a phosphorescent emissive layer comprising a second hostmaterial; a partial hole-blocking layer having a first thicknessdisposed between the fluorescent emissive layer and phosphorescentemissive layer; wherein the fluorescent emissive layer and thephosphorescent emissive layer define a recombination zone having asecond thickness; and wherein the first thickness is less than aboutone-third of the second thickness.
 2. The top-emission white OLEDaccording to claim 1, wherein the first thickness is in the range of 0.5nm to 3 nm.
 3. The top-emission white OLED according to claim 1, whereinthe partial hole-blocking layer allows about 50% to about 95% of theholes reaching the hole blocking layer to pass from the phosphorescentemissive layer to the fluorescent emissive layer, or allows about 50% toabout 95% of the holes reaching the hole blocking layer to pass from thefluorescent emissive layer to the phosphorescent emissive layer.
 4. Thetop-emission white OLED according to claim 1, wherein both thefluorescent emissive layer and the phosphorescent emissive layer are 20nm thick.
 5. The top-emission white OLED according to claim 1, wherein ahighest occupied molecular orbital of the partial hole-blocking layerhas a higher energy than a highest occupied molecular orbital of thefirst host material.
 6. The top-emission white OLED according to claim1, further comprising an anode and a cathode, wherein the emissiveconstruct is positioned between the anode and the cathode.
 7. Thetop-emission white OLED according to claim 6, further comprising: ahole-injection layer disposed over the anode; a hole-transport layerdisposed over the hole-injection layer; wherein the emissive constructis over the hole-transport layer; an electron transporting layerdisposed over the emissive construct; an electron injection layerdisposed on the electron transporting layer; and wherein the cathode isdisposed on the electron transport layer.
 8. A top-emission whiteorganic light-emitting diode (OLED) comprising an emissive construct,wherein the emissive construct comprises: a fluorescent emissive layercomprising a first host material that emits blue light and a bluelight-emitting fluorescent dopant; a phosphorescent emissive layercomprising a second host material; a partial hole-blocking layer havinga first thickness disposed between the fluorescent emissive layer andphosphorescent emissive layer; wherein the fluorescent emissive layerand the phosphorescent emissive layer define a recombination zone havinga second thickness; and wherein the first thickness is less than aboutone-third of the second thickness, and a highest occupied molecularorbital of the partial hole-blocking layer has a higher energy than ahighest occupied molecular orbital of the first host material.
 9. Thetop-emission white OLED according to claim 8, where the first hostmaterial has a T1that is lower than a T1of the blue light-emittingfluorescent dopant.
 10. The top-emission white OLED according to claim8, where the first host material has a S1energy level that is higherthan a S1energy level of the blue light-emitting fluorescent dopant. 11.The top-emission white OLED according to claim 8, wherein both thefluorescent emissive layer and the phosphorescent emissive layer are 20nm thick.
 12. The top-emission white OLED according to claim 8, furthercomprising an anode and a cathode, wherein the emissive construct ispositioned between the anode and the cathode.
 13. The top-emission whiteOLED according to claim 12, further comprising: a hole-injection layerdisposed over the anode; a hole-transport layer disposed over thehole-injection layer; wherein the emissive construct is over thehole-transport layer; an electron transporting layer disposed over theemissive construct; an electron injection layer disposed on the electrontransporting layer; and wherein the cathode is disposed on the electrontransport layer.
 14. A top-emission white organic light-emitting diode(OLED) comprising: an emissive construct, wherein the emissive constructcomprises: a fluorescent emissive layer comprising a first hostmaterial; a phosphorescent emissive layer comprising a second hostmaterial, a yellow-emitting phosphorescent dopant, and a red-emittingphosphorescent dopant; a partial hole-blocking layer having a firstthickness disposed between the fluorescent emissive layer andphosphorescent emissive layer; wherein the fluorescent emissive layerand the phosphorescent emissive layer define a recombination zone havinga second thickness; wherein the first thickness is less than aboutone-third of the second thickness; and wherein the light emitted by thephosphorescent dopant provides white light when combined with the lightemitted by the fluorescent emissive layer an anode and a cathode,wherein the emissive construct is positioned between the anode and thecathode; a hole-injection layer disposed over the anode; ahole-transport layer disposed over the hole-injection layer; wherein theemissive construct is over the hole-transport layer; an electrontransporting layer disposed over the emissive construct; an electroninjection layer disposed on the electron transporting layer; and whereinthe cathode is disposed on the electron transport layer.
 15. Thetop-emission white OLED according to claim 14, wherein thephosphorescent emissive layer further comprises a first sub-layer and asecond sub-layer, wherein the first sub-layer comprises theyellow-emitting phosphorescent dopant and the second sub-layer comprisesthe red-emitting phosphorescent dopant.
 16. The top-emission white OLEDaccording to claim 14, wherein a highest occupied molecular orbital ofthe partial hole-blocking layer has a higher energy than a highestoccupied molecular orbital of the first host material.